kwired
Electron manager
- Location
- NE Nebraska
You have a separately derived system whether your "load" requires or neutral or not.
The X0 in bonded to the enclosure and you ground the X0 as directed by the NEC. The line conductors and a EGC is pulled to the load. Should there be a L-G fault the fault current will flow back to the X0 causing the OCPD to trip. Since the secondary is a 208Y/120 any L-G ground fault will be 120v regardless if there is a neutral pulled to a load or not. To even consider grounding a phase conductor in leu of the X0 does not make sence. If one elects to ground one of the 208v lines a ground fault will now be 208v. What anvantage is that? Is that better than 120v? When there is no choice other than grounding a phase such as corner grounding a 240 or 480v delta I find it unreasonable to even consider grounding one of the phases of a wye. One of the guys said "Just because you can doesn't mean you should."
If you have a 480 volt corner grounded system and need a backup generator would you say it is unsafe to use a generator with wye connected windings, float the neutral and ground a phase? I would not have a problem with it.
Why would the situtation with the transformer be any different.
Alternating-Current Systems to Be Grounded. 250.20(B)(2):
Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
If the neutral conductor is not used as a circuit conductor it does not have to be the conductor that is grounded. It is usually wise to still use it as the grounded conductor if that is possible.